Keeping voltage under control

02-22-2016 - The following rule applies when electricity generation systems feed electricity into the grid: more power leads to a higher voltage. But the voltage range that the operator has to comply with is limited. Voltage may only deviate by ten per cent from its nominal value. When the sun shines and the wind blows, photovoltaic systems and wind turbines deliver their full output. Grid voltage increases if any consumers are missing at that time. Voltage range violations can be a result.

Although energy can be transmitted to higher-level grids via substations, distribution network operators have to ensure compliance with voltage tolerance limits. Conventional grid expansion is very expensive. For this reason, Green Access is looking for alternative solutions. The aim: A distribution network that autonomously adapts to future load and feed-in changes, and varying grid topologies. Before the control scheme can be tested on a real grid, which would be the technical equivalent of open-heart surgery, the function is first being tested in simulations. The project partners developed five simulation scenarios to obtain a reliable result, which have now produced some preliminary results.

Overview of the Green Access simulation scenarios

The flow chart shows how control takes place within the different scenarios. Image: Green Access, Fraunhofer ISE

Scenario 0 – status quo

In this scenario, there is no control scheme for voltage stabilisation or reactive power provision by decentralised electricity generation systems. All feed-in systems are static and deliver their output according to supply.

Scenario 1 – local component control

In this scenario, the individual energy generation systems have their own controllers and attempt to maintain a stable voltage at their respective feed-in points. The decentralised controllers operate autonomously following specified and fixed statics, which means they are not interconnected.

Scenario 2 – centralised low voltage control

Again, the grid has a large number of decentralised controllers but in contrast to their autonomous function in Scenario 1, this calculation adds communication between the controllers in a low voltage segment. However, there is no communication between the medium voltage and low voltage level.

Scenario 3 – centralised medium voltage control

In the last calculation, the controllers are fully networked within the grid. This means that the decentralised controllers are networked in the low voltage as well as the medium voltage grids. Unlike Scenario 2, there is a central exchange between the voltage levels.

Scenario 4 - comparative scenario with grid expansion

The researchers demonstrate how effective the scenarios one through three are using a comparative analysis. For this comparison, they assume the configuration of Scenario 0 and calculate the need for conventional grid expansion.

The aim: maximum active power with optimal voltage stability

In Scenarios 1 through 3, decentralised energy generation systems provide reactive power to maintain voltage stability. With their calculations, the project partners can estimate the extent to which new control schemes are suitable for voltage stabilisation in low and medium voltage grids even with a large number of decentralised electricity generation systems. The result shows that the grid voltage exceeds its permissible limits under certain conditions. While this only happens on rare occasions, a further increase of wind and solar power would aggravate the problem. To illustrate this, the project partners calculated Scenario 0 with an additional 294 kilowatts from photovoltaic systems. The result: The voltage was too high at nearly half of all grid connection points.

Merely using decentralised controllers greatly reduced the number of voltage range violations. The use of controllable secondary substation transformers (rONT) and generation systems with an output control for reactive power is worthwhile from a technical perspective. Further grid expansion is not required, or at least to a far lesser extent. Can this result be improved? Voltage remains within limits when using local component controllers. In the centrally organised control scheme of the third scenario, the permissible voltage range can be utilised up to the upper limit but reactive power provision for voltage stabilisation dropped by up to 27 per cent when compared to the use of local controllers. This allows for improved efficiency since grid losses are lowered. With this combined control of low and medium voltage levels in the framework of Green Access, the aim is to demonstrate that an optimum of physical effectiveness (compliance with the permissible voltage range) and efficiency of control (minimising reactive power) can indeed be achieved.